With the rapid advance of radio communication technology, the communication systems have highly evolved, and Long Term Evolution (LTE) system of the 3rd Generation Partnership Project (3GPP) is one of the promising 4th Generation (4G) mobile communication systems.
FIG. 1 is a diagram illustrating an architecture of an LTE system according to the related art.
Referring to FIG. 1, a radio access network of the LTE system includes evolved Node Bs (eNBs) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130. The User Equipment (hereinafter, referred to as a UE) 135 connects to an external network via the eNBs 105, 110, 115, and 120 and the S-GW 130.
Referring to FIG. 1, the eNBs 105, 110, 115, and 120 correspond to the legacy node Bs of the UMTS system. The eNBs 105, 110, 115, and 120 allow the UE 135 to establish a radio channel and are responsible for functions more complicated as compared to the legacy node B. In the LTE system, all the user traffic services including real time services, such as Voice over Internet Protocol (VoIP), are provided through a shared channel and thus there is a need of a device to schedule data based on the state information (such as buffer status, power headroom status, and channel condition of the UE), the eNBs 105, 110, 115, and 120 being responsible for such functions. Typically, one eNB controls a plurality of cells. In order to secure the data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. In addition, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE. The S-GW 130 is an entity to provide data bearers so as to establish and release data bearers under the control of the MME 125. The MME 125 is responsible for mobility management of UEs and various control functions and may be connected to a plurality of eNBs.
FIG. 2 is a diagram illustrating a protocol stack of an LTE system according to the related art.
Referring to FIG. 2, the protocol stack of the LTE system includes Packet Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control (RLC) 210 and 235, Medium Access Control (MAC) 215 and 230, and Physical (PHY) 220 and 225. The PDCP 205 and 240 is responsible for IP header compression/decompression, and the RLC 210 and 235 is responsible for segmenting the PDCP Protocol Data Unit (PDU) into segments in appropriate size for Automatic Repeat Request (ARQ) operation. The MAC 215 and 230 is responsible for establishing connection to a plurality of RLC entities so as to multiplex the RLC PDUs into MAC PDUs and demultiplex the MAC PDUs into RLC PDUs. The PHY 220 and 225 performs channel coding on the MAC PDU and modulates the MAC PDU into OFDM symbols to transmit over radio channel or performs demodulating and channel-decoding on the received OFDM symbols and delivers the decoded data to the higher layer. In addition, the PHY layer uses Hybrid ARQ (HARQ) for additional error correction by transmitting 1 bit information indicating for positive or negative acknowledgement from the receiver to the transmitter. This is referred to as HARQ ACK/NACK information. The downlink HARQ ACK/NACK corresponding to the uplink transmission is carried by Physical Hybrid-ARQ Indicator Channel (PHICH), and the uplink HARQ ACK/NACK corresponding to downlink transmission is carried by Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).
Meanwhile, a new transmission scheme known as dual connectivity which is capable of allowing a UE to communicate multiple eNBs simultaneously is being developed as a part of LTE. A dual connectivity-capable UE may transmit data to and receive data from different eNBs simultaneously. For example, the dual connectivity-capable UE may connect to a macro eNB having a relatively large coverage area and a pico eNB having a relatively small coverage simultaneously. In this case, the UE can communicate with the pico eNB at a high data rate while, if its mobility is low, maintaining the mobility through connection with the macro eNB.
In order for the UE to communicate data with multiple eNBs simultaneously as described above, the UE has to perform the random access with respective eNBs. The random access procedure is performed to acquire uplink synchronization with the eNB for data transmission, and the UE incapable of the dual connectivity can perform the random access procedure with only one eNB. However, the UE capable of the dual connectivity may perform the random access procedure with multiple eNBs independently and thus there is a need of a random access method capable of allowing the UE to perform the random access procedure with two or more eNBs simultaneously based on the transmit power constrain of the UE.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.